The present invention relates in general to materials handling vehicles and, more particularly, to a device for monitoring movement of mast elements of such vehicles so that the height of a mast, an associated load lifting device, an operator""s platform or the like can be determined.
The importance of determining the height of masts, load lifting devices, such as forks, operator""s platforms and the like, generally referred to herein as xe2x80x9cmast heightxe2x80x9d, is well known in the art. Known mast height sensing or measuring devices have taken a wide variety of forms.
For example, a float actuated potentiometer monitoring the liquid level in a sump tank of a hydraulic system controlling mast extension to determine mast height is disclosed in U.S. Pat. No. 4,598,797.
A disc coupled to a chain wheel used for controlling a mast and having a plurality of slits which pass through a light emitting/detecting path with resulting pulse signals being counted to determine mast height is disclosed in U.S. Pat. No. 4,499,541.
In EP 0 335 196 A1, a gear coupled to a resolver mounted on a stationary upright of a mast assembly is driven by a ladder assembly mounted on a movable upright of the mast assembly and having rungs or teeth engaging and rotating the gear so that the resolver generates a signal representative of mast height.
The height and speed of a carriage elevated by a screw lift is monitored by a rotary encoder that senses rotary angular displacement of the screw in U.S. Pat. No. 4,782,920.
In U.S. Pat. No. 5,022,496, the extension and retraction of a cable wound on a spring biased take-up reel mounted on a platform assembly of a turret stockpicker activates an encoder that produces output pulses in direct relation to the amount of rotation of the reel so that the vertical position of the platform assembly can be determined by a microcomputer receiving the pulses.
In U.S. Pat. No. 3,319,816, direction and distance of movement of a moving mast member of a lift truck relative to a fixed mast member of the truck is measured using a transducer secured to the fixed mast member. The transducer includes a potentiometer that is rotated through a gear train extending between the potentiometer and a friction wheel that engages and is rotated by movement of the moving mast member.
In GB 2 156 099A, a mast height sensor is disclosed wherein a rotary shaft encoder is driven by a wheel having a rubber tire mounted thereon that is spring biased against a mast member so that the wheel and hence the shaft of the encoder are rotated by relative movement between the mast members. The encoder generates pulses for predetermined degrees of rotation in either direction and by counting these pulses up and down a measure of mast/platform height is derived.
In U.S. Pat. No. 6,269,913 B1, a mast height sensor uses a roller bearing with a built in sensor for determining the speed and/or relative displacement of the outer race of the bearing relative to the inner race of the bearing. The inner race of the bearing is fixed to a first mast member and the outer race is elastically preloaded against a second mast member to serve as a roller body as the mast members move relative to one another. Rotation of the outer race relative to the inner race is monitored using signals generated by the built-in sensor which signals are counted and used in a conventional quadrature direction sensing arrangement to determine direction of movement, mast height and speed of mast movement.
Unfortunately, many of these mast height sensors lack the accuracy required for modern day materials handling vehicle operating systems. Others do not hold up under operating conditions encountered by many materials handling vehicles. Still others do not operate properly when they encounter severe operating conditions. For example, mast height sensor problems have been experienced when materials handling vehicles are operated in big freezers in food warehouses that can be operated at temperatures as low as xe2x88x9240xc2x0 F. (xe2x88x9240xc2x0 C.). Even if a sensor can tolerate such cold temperatures, the vehicles move from the freezers to warmer rooms and/or outside so that condensation forms on the sensors and mast assemblies with the condensation often being in the form of ice. Such operating conditions are a particular problem for mast height sensors that rely on frictional engagement of a rotating member, such as a rubber covered wheel or an outer race of a bearing sensor, since moisture and ice reduce the friction necessary for their operation. Thus, frictional contacts that may work perfectly well in normal room and warehouse temperatures, fail when operated in and around freezers. Thus, there is a need for an improved mast height sensor for materials handling vehicles that can operate not only under normal operating conditions but also in adverse conditions such as those encountered in and around freezers.
This need is met by the invention of the present application wherein a knurled wheel is coupled to a sensor mounted to one of at least two mast members and the wheel is forced into contact with another mast member so that the wheel is rotated when the mast members are moved relative to one another. Rotation of the wheel causes the sensor to generate signals corresponding to the movement of the mast members. The signals generated by the sensor are processed conventionally to determine mast height, direction of movement of one or more of the mast members, speed of movement of one or more of the mast members and acceleration of one or more of the mast members, as needed. The knurled outer periphery of the wheel is forced into the mast member that it contacts with sufficient force so that a track corresponding to the knurl on the wheel is formed in the contacted mast member and the knurl engages the track for better traction. In a working embodiment of the invention, the wheel was forced into the contacted mast member with a force of six to nine pounds. To enhance the versatility of the mast height sensing device of the present application, the thickness of the wheel can be made less than xe2x85x9 inch, for example {fraction (1/16)} inch. Use of a thin wheel enhances operation of the mast height sensor in cold environments, such as food freezers of warehouses where ice may form on the contacted mast member. The sensor can be an encoder or a sensor bearing and a heater can be provided for operation in cold environments to ensure rotation of the sensor under such conditions.
A variety of features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.